It is well-known that laser glasses must be able co withstand high internal temperatures created by flashlamps and the like without experiencing significant distortion, cracking or changes in optical properties that would significantly degrade the optical properties of the resonator during operation. Commonly referred to as thermal shock resistance, optical elements such as laser rods, slabs, discs, and fibers must also be capable of enduring high frequency and/or high average power optical pumping without catastrophic failure.
As will be appreciated by those skilled in the art, operation a laser glass element acquires heat from the pumping light source. In order to dissipate this heat, laser devices are typically liquid-cooled to maintain the laser glass element within its operating temperature range. This cooling of the laser element creates a thermal gradient resulting in thermally-induced stress within the element that in turn induces optical distortions. If the thermally-induced stress exceeds the rupture strength of the glass, catastrophic failure of the laser glass element occurs. It is also known that under repetitively pulsed or continuously pumped conditions, conventional phosphate laser glass fractures and/or exhibits optical distortion at relatively low power levels which severely restricts the range of applications for these materials. Typically, athermal phosphate laser glass compositions have low rupture strength. This limits their use to average power levels of about 50% of that which may be employed with silicate glass compositions. Although their high rupture strength is desirable, silicate laser glass compositions do not compare favorably with athermal phosphate laser glass compositions in two respects: (1) silicate glass compositions have lower gain than the phosphate laser glass compositions; and (2) silicate glass compositions are not athermal. Moreover, these limitations of silicate glasses combine to effectively restrict their use to power levels comparable to those utilized with in phosphate laser glass compositions, notwithstanding that the silicates demonstrate twice the rupture strength.
Efforts to improve the rupture strength of both silicate and phosphate laser glasses by ion-exchange methods are discussed U.S. Pat. Nos. 3,687,799 and 5,053,360 which have been assigned to the assignee of the present invention. The methods of improving rupture strength described therein have found important uses, but the resultant glasses are limited in some applications by optical constraints due to the presence of a compressive surface layer. It is known that the compressive surface layer has a highly reflective internal surface that adversely affects performance and manufacturing costs.
The present invention provides phosphate glass compositions which satisfy the need for athermal performance at high average power levels with the concurrent high rupture strength, high gain, and good chemical durability necessary for superior performance without the need for ion exchange or other surface treatment methods which may degrade performance and increase cost.
Other patents which disclose phosphate glass compositions are U.S. Pat. Nos. 5,053,165 (Oct. 1, 1991); 5,032,315 (Jul. 16, 1992); 4,929,387 (May 29, 1990); 4,871,230 (Oct. 3, 1989); 4,820,662 (Apr. 11, 1989); 4,333,848 (Jun. 8, 1992); and 4,022,707 (May 10, 1977). None of these patents, however, disclose the laser glass compositions of the present invention having the unexpected properties described herein.